> In New Zealand, Hebe species can be found growing in a wide range of habitats, from sea level to alpine regions, so it is no surprise that cold hardiness of the species, and the cultivars derived from them, varies widely as well. There is truth to the old saying that hardiness of Hebe is related to leaf size. As one goes up in elevation from sea level to alpine areas in New Zealand, the leaf size of the Hebes tends to decrease, and overall plant size decreases as well (Kristensen, 1989). Other characteristics like coriaceous and/or glacous leaves and white flowers are also typical of alpine Hebe species (Wardle, 1978). So, generally speaking, you could say that the larger the leaf of the Hebe, the less cold hardy it tends to be. As with all living things, the rule is not perfect, but the most tender Hebes are usually the largest-leaved, and the hardiest are those with the smallest leaves.
> The most extensive study of cold hardiness of Hebe was undertaken by Warrington and Southward (1995), who assessed summer and winter hardiness of 35 species and cultivars. This study showed that large differences in hardiness existed among the various selections. Not surprisingly, the hardiest of those tested were two whipcords, H. cupressoides and H. propinqua, both of which are typically found in alpine or subalpine regions and have very tiny leaves. More important than altitude however, this study observed significant differences in hardiness of species from northern or southern parts of New Zealand, with species of southern origin exhibiting greater overall hardiness.
> Some of the research on hardiness of Hebe has utilized excised shoots as sample material for laboratory studies. Bannister (1986) found that detached shoots of Hebe albicans, a South Island species often found in subalpine scrub above 1000m, withstood mid-winter temperatures of -10°C, one of the hardier of the native species studied. Other Hebe species were not tested. In a more extensive study, Bannister (1990) found that mid-winter freezing resistance of foliage of H. buchananii, a diminutive shrub of alpine areas in Canterbury, was as low as -11°C. Freezing resistance of foliage of H. rakaiensis and H. salicifolia, both of which are found at lower elevations on the South Island, was found to be -5.2°C and -6°C respectively. BuPost too long. Click here to view the full text.